In conventional hologram transfer films, in general, a surface relief hologram layer, wherein a hologram is formed by providing concaves and covexes on the surface thereof, is mainly used in a hologram-forming layer. A representative construction of the hologram transfer film known in the art comprises a substrate film having thereon a release layer, a surface relief hologram layer, a reflective thin film layer, and a heat-sensitive adhesive layer.
At the time of thermal transfer, in other words, hot stamping, the hologram layer in the hologram transfer film should be sharply cut and transferred onto an object, that is, should have good transferability and should be exactly transferred onto an object.
Many of the conventional hologram transfer films, however, have unsatisfactory hologram transferability, and are very unstable in transfer process. That is, the hologram layer can be in some cases satisfactorily transferred and in some cases cannot be satisfactorily transferred. Therefore, an improvement in the transferability has been desired in the art.
Accordingly, it is a first object of the present invention to provide a hologram transfer film which can solve the above problem of the prior art and enables a hologram to be easily and exactly transferred with good stability in the transfer process.
An example of a conventional forgery-preventive sheet is such that a forgery-preventive printed pattern (a ground design) is formed by printing fine dots or hairlines on the surface of the sheet. In this sheet, upon copying in a light mode, the dots or the lines cannot be clearly copied, while upon copying in a deep mode, the dots are blurred and seen like lines or letters, and the lines are blurred to thicken the lines. In recent years, however, this method has become insufficient for preventing forgery.
In order to solve this problem, a forgery-preventive sheet has been proposed wherein, for example, a lustrous thread is infused into one side of a base sheet (Japanese Patent Laid-Open No. 56377/1995). This forgery-preventive sheet is intended to prevent forgery by utilizing a phenomenon such that, even upon color copying, a metallic color does not appear in portions corresponding to the lustrous thread.
Further, diffraction gratings or relief holograms have hitherto been used for the decoration or the prevention of forgery of various cards, securities and the like.
With expansion of the above applications, the diffraction grating or the relief hologram has become required to have such properties as the capability of being infused as watermarks into papers, excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates, and conformability to flexing or expansion/contraction of objects with holograms or diffraction gratings being applied thereon. In particular, in the case of diffraction gratings or relief holograms formed of a cured product of a conventional photocurable coating composition, a resin used in the production thereof loses flexibility upon photocuring, and, thus, flexing or expansion/contraction of an object with the holograms or the diffraction grating being applied thereon results in loss of the function of the diffraction grating or the relief hologram.
Accordingly, it is a second object of the invention to provide a photocurable resin composition that can form a diffraction grating, a hologram or the like which can solve the problems of the prior art and has excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates, and conformability to flexing or expansion/contraction of objects with holograms or diffraction gratings being applied thereon, and to provide a hologram or diffraction grating sheet.